In gas sensor applications, the arrangement of the sensor elements on their underlying substrate should exhibit certain attributes to improve or optimize their performance. In particular, it is desirable to physically arrange and integrate the sensing and operational elements of the sensor so that the components are maintained at essentially the same temperature. In practice, the sensor elements should be arranged so as to minimize the substrate area occupied by the elements, thereby reducing or minimizing thermal convection and conduction losses among the sensor elements. Secondarily, minimizing the occupied substrate area also reduces the amount of substrate material required to fabricate the sensor, and thus reduces fabricating costs.
Conventional, prior art solutions, such as those developed at Sandia National Laboratories (see R. Thomas and R. Hughes, “Sensors for Detecting Molecular Hydrogen Based on PD Metal Alloys”, J. Electrochem. Soc., Vol. 144, No. 9, September 1997; and U.S. Pat. No. 5,279,795), involve the interlacing of the sensing and operational elements of the sensor. Such conventional solutions employ a geometry that deploys the sensor elements over a significantly larger area than necessary, thus rendering the design less effective in terms of thermal layout, in that the sensing element(s) of the sensor (capacitive metal-on-silicon (MOS) elements in the case of the Sandia design) do not occupy a common, uniform thermal environment. The Sandia design also has greater than optimal manufacturing costs due to the interlaced design rendering unused significant portions of the underlying substrate material. The Sandia and similar prior art designs did not seek to optimize the thermal environment of the sensor assembly. Nor did the Sandia or similar prior art designs seek to optimize the mechanical compactness of the sensor assembly.
Although conventional, prior art solutions had some thermal integration of the heating element, the temperature sensor and the gas sensor, the geometry was such that these elements could become flow sensitive. Flow sensitivity refers to the effect that the flow rate of the gas stream to be measured can have in conducting heat from the element(s), thereby lowering their temperature and requiring additional electrical power to restore the temperature of the element(s) to their original and desired level. Sandia-type sensor designs included an additional capacitive (MOS) sensor, which was located outside of the portion of the assembly having controllable and uniform thermal properties. Moreover, such conventional, prior art designs had considerable wasted space on the underlying silicon die or substrate on which the sensor elements were arranged, which would multiply (approximately triple) the manufacturing costs and heat loss from the sensor elements to the external environment.
In the present gas sensor assembly, the sensing and control elements are mounted on the underlying substrate and operated so as to maintain thermal integrity and prevent heat loss. In particular, the area occupied by the sensor elements is minimized or conserved to minimize or reduce thermal losses to convection/conduction of heat to and from the sensor components. The present gas sensor assembly is also configured for compactness to minimize or reduce manufacturing and parts costs by maximizing or increasing the number of sensor elements mounted on a substrate.